Half-bridge circuit, power supply device, and method for driving half-bridge circuit

ABSTRACT

In a half-bridge circuit, in a case that a first transistor element is turned ON, a primary winding current flows from a power supply to a primary winding. Then, in a case that the first transistor element is turned OFF, (i) a first rectifying element current flows from a secondary winding to a first rectifying element, or (ii) a second rectifying element current flows from a tertiary winding to a second rectifying element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese PatentApplication Number 2018-234917 filed on Dec. 14, 2018. The entirecontents of the above-identified application are hereby incorporated byreference.

BACKGROUND Technical Field

The following disclosure relates to a half-bridge circuit.

In a half-bridge circuit used in a power supply circuit, it is knownthat a reverse recovery current (also referred to herein as a transientcurrent) is generated. This transient current is generated by applying avoltage that inhibits a rectified current when a rectified current isflowing to a switching element such as a Metal Oxide Semiconductor FieldEffect Transistor (MOSFET). This transient current produces a loss inthe circuit, and thus various countermeasure methods have been studied.

JP 2011-36075 A and JP 2013-198298 A disclose circuits that are intendedto reduce transient current. For example, in the circuit disclosed in JP2011-36075 A, a diode and a transformer connected in parallel to aswitching element are provided to reduce transient current. In JP2013-198298 A as well, a circuit similar to that in JP 2011-36075 A isdisclosed.

However, as described later, there is still room for improvement inreducing the transient current in a half-bridge circuit. An object of anaspect of the present disclosure is to effectively reduce transientcurrent in a half-bridge circuit.

SUMMARY

To solve the above-described problem, a half-bridge circuit according toan aspect of the present disclosure includes a first switching elementconnected to a first terminal serving as a high-voltage node and asecond terminal serving as a switch node and a second switching elementconnected to a third terminal serving as a switch node and a fourthterminal serving as a low-voltage node. The half-bridge circuit furtherincludes a transformer including a primary winding, a secondary winding,and a tertiary winding, a first rectifying element connected in parallelwith the first switching element with the secondary winding interposedbetween the first rectifying element and the first switch element, asecond rectifying element connected in parallel with the secondswitching element with the tertiary winding interposed between thesecond rectifying element and the second switch element, a firsttransistor element connected to the primary winding, and a power supplyconnected to the primary winding. In a case that the first transistorelement is turned ON, a primary winding current, serving as a currentflowing from the power supply to the primary winding, flows, and in acase that the first transistor element is turned OFF, (i) a firstrectifying element current, serving as a current flowing from thesecondary winding to the first rectifying element, flows, or (ii) asecond rectifying element current, serving as a current flowing from thetertiary winding to the second rectifying element, flows.

Further, to solve the above-described problem, a method for driving ahalf-bridge circuit according to an aspect of the present disclosure isa method for driving a half-bridge circuit including a first switchingelement connected to a first terminal serving as a high-voltage node anda second terminal serving as a switch node and a second switchingelement connected to a third terminal serving as a switch node and afourth terminal serving as a low-voltage node. The half-bridge circuitfurther includes a transformer including a primary winding, a secondarywinding, and a tertiary winding, a first rectifying element connected inparallel with the first switching element with the secondary windinginterposed between the first rectifying element and the first switchingelement, a second rectifying element connected in parallel with thesecond switching element with the tertiary winding interposed betweenthe second rectifying element and the second switching element, a firsttransistor element connected to the primary winding, and a power supplyconnected to the primary winding. The method includes a first step forapplying a forward voltage to the first switching element and causing arectified current to flow to the first switching element, a second stepfor turning the first transistor element ON after the first step andcausing a primary winding current, serving as a current flowing from thepower supply to the primary winding, to flow, a third step for turningthe first transistor element OFF after the second step and causing afirst rectifying element current, serving as a current flowing from thesecondary winding to the first rectifying element, to flow, and a fourthstep for applying a reverse voltage to the first switching element afterthe third step within a time period when the first rectifying elementcurrent is flowing.

According to the half-bridge circuit according to an aspect of thepresent disclosure, it is possible to effectively reduce transientcurrent. Further, according to the method for driving the half-bridgecircuit according to an aspect of the present disclosure, the sameeffects can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a diagram illustrating a circuit configuration of a powersupply circuit according to a first embodiment.

FIG. 2 is a diagram showing waveforms of each voltage and current.

FIG. 3 is a diagram showing each graph of FIG. 2 enlarged.

FIGS. 4(a) to 4(d) are diagrams for explaining the path of each currentin a first step to a fourth step in a first time period, respectively.

FIG. 5 is a diagram showing waveforms of rectifier function unit voltageand rectifier function unit current in a power supply circuit of acomparative example.

FIGS. 6(a) to 6(d) are diagrams for explaining the path of each currentin a first step to a fourth step in a second time period, respectively.

FIG. 7 is a diagram illustrating a circuit configuration of a powersupply circuit according to a second embodiment.

FIG. 8 is a diagram illustrating a circuit configuration of a powersupply circuit according to a third embodiment.

FIG. 9 is a diagram illustrating a power supply device according to afourth embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

A half-bridge circuit 1 of the first embodiment will be described below.Note that, for convenience of description, in each embodimenthereinafter, components having the same functions as those of componentsdescribed in the first embodiment are denoted using the same referencenumerals, and descriptions thereof will not be repeated.

Purpose of Half-Bridge Circuit 1

As described above, a transient current flows in a half-bridge circuit.Transient current is known to be mainly generated in a switching elementwith a PN junction.

Meanwhile, examples of semiconductor elements without a PN junctioninclude a SiC Schottky Barrier Diode (SBD), a GaN High Electron MobilityTransistor (HEMT), and the like. In these semiconductors, transientcurrent originating from the PN junction does not occur. However, theseswitching elements are provided with parasitic capacitance. As a result,there is a problem in that, in association with the application ofvoltage for inhibiting the rectified current, a charging current(transient current) of the parasitic capacitance flows. The half-bridgecircuit 1 is created for the purpose of reducing these transientcurrents.

Definitions of Terms

Prior to describing the half-bridge circuit 1, each term is definedherein as follows.

The term “forward voltage” refers to a voltage for causing a forwardcurrent to flow to a rectifying element. Consider, as a first example, acase in which the rectifying element is a diode. In this case, “forwardvoltage” refers to the voltage applied to cause a forward current toflow to the diode.

Consider, as a second example, a case in which the rectifying element isa MOSFET, a GaN-HEMT, or the like. That is, consider a case in which therectifying element includes a gate, a source, and a drain. In this case,the forward voltage refers to the “voltage allowing current to flow inthe rectifying element in a case in which a positive voltage is appliedto the source based on the drain when the gate is OFF (the gate voltageis less than a threshold voltage).”

These two examples are the same as applying a positive voltage to ST1based on FT1 of FIG. 1 described later. The size of the forward voltagedepends on the type of element, but is, for example, from 0.1 V to 5 V.The size of the current that occurs in association with application ofthe forward voltage depends on the current of the inductive element,such as a coil, but is, for example, from 0.1 A to 100 A.

The term “rectified current” refers to a forward current flowing in arectifying element. In the example of FIG. 1, the rectified current canbe measured at FS1 and SS1 as well as TS1 and PS1 described later.

The term “reverse voltage” refers to a voltage applied to the rectifyingelement so that a forward current does not flow. Consider, as a firstexample, a case in which the rectifying element is a diode. In thiscase, the voltage applied so that a forward current does not flow to thediode is the reverse voltage.

Consider, as a second example, a case in which the rectifying element isa MOSFET, a GaN-HEMT, or the like. In this case, the reverse voltagerefers to the “positive voltage applied to the drain based on the sourcewhen the gate is OFF (the gate voltage is less than a thresholdvoltage).” The two examples described above are the same as applying apositive voltage to FT1 based on ST1 of FIG. 1. Further, the twoexamples described above are the same as applying a positive voltage toTT1 based on PT1 of FIG. 1. The size of the reverse voltage depends oncircuit specifications, but is, for example, from 1 V to 1200 V.

The term “transient current” generally refers to a reverse recoverycurrent and a charging current of the parasitic capacitance of arectifying element. That is, the transient current refers to a transientcurrent that occurs in a case that a reverse voltage is applied to arectifying element. In the example of FIG. 1, the transient current canbe measured at FS1 and SS1 as well as TS1 and PS1.

The term “rectifier function” refers to a function that conducts only acurrent flowing in one direction and does not conduct a current in adirection opposite to that direction. Consider, as a first example, acase in which the rectifying element is a diode. In this case, therectifier function refers to the function of the diode conducting theforward current and blocking the reverse current.

Consider, as a second example, a case in which the rectifying element isa MOSFET, a GaN-HEMT, or the like. In this case, the rectifier functionrefers to the function of the rectifying element that conducts currentfrom the source to the drain and blocks current from the drain to thesource at gate OFF. In a case that the rectifying element is a MOSFET ora GaN-HEMT, (i) the source can be considered the anode of the diode, and(ii) the drain can be considered the cathode of the diode for therectifier function.

The term “rectifying element” generally refers to an element having arectifier function. Each of the diodes, MOSFETs, and GaN-HEMTs describedabove is an example of a rectifying element. In a case that therectifying element is a diode, the (i) cathode is connected to FT1 ofthe rectifier circuit, and (ii) the anode is connected to ST1 of therectifier circuit. Note that the term “connection” herein refers to an“electrical connection” unless expressly stated otherwise.

Note that an element (including a winding of a transformer as well) maybe interposed as necessary in (i) the connection between the cathode andFT1 and (ii) the connection between the anode and ST1.

The term “transistor function” refers to a function capable ofdetermining whether a current flows by ON/OFF of the gate only. In acase that the element is a MOSFET, a GaN-HEMT, or the like, thetransistor function refers to the function of switching the currentflowing from the drain toward the source by ON/OFF of the gate. Ofcourse, to cause current to flow, it is also necessary to apply apositive voltage to the drain based on the source.

Note that in a case that the element is a bipolar transistor, anInsulated Gate Bipolar Transistor (IGBT), or the like, (i) the drain canbe considered a collector, and (ii) the source can be considered anemitter.

The term “transistor element” generally refers to an element having atransistor function. As described above, a MOSFET, a GaN-HEMT, a bipolartransistor, an IGBT, and the like are transistor elements because theyhave a transistor function.

The term “switching element” generally refers to an element having arectifier function and a transistor function. As described above, aMOSFET, a GaN-HEMT, and the like are switching elements because theyhave both the rectifier function and the transistor function. Withregard to GaN-HEMTs, both enhancement mode and cascode type areswitching elements.

On the other hand, a typical IGBT has a transistor function but does nothave a rectifier function. Thus, to use an IGBT as a switching element,generally an IGBT provided with a reverse parallel diode is used. TheIGBT applied to the switching elements described herein is defined as anIGBT of this type.

The term “rectifier function unit,” in a case that one side arm (anupper arm side or a lower arm side) illustrated in FIG. 1 serves as arectifier function, refers to a section in which a switching element, arectifying element, and a transformer winding of the arm are formed as aset.

The term “transistor function unit” refers to a section that functionsas a transistor. The term has the same meaning as transistor element. Arectifying element and a transformer winding are not included.

The term “first time period” refers to a time period in which the upperarm side is used as the rectifier function unit and the lower arm sideis used as the transistor function unit. The length of the first timeperiod is defined as the time period in which at least one cycle of aswitching operation is repeated. As an example, the first time period isa time period during which a step-up operation of a bidirectional DC/DCconverter is performed (step-up operation time period).

The term “second time period” refers to a time period in which the lowerarm side is used as the rectifier function unit and the upper arm sideis used as the transistor function unit. The length of the second timeperiod is defined as the time period in which at least one cycle of aswitching operation is repeated. As an example, the second time periodis a time period during which a step-down operation of a bidirectionalDC/DC converter is performed (step-down operation time period).

Overview of Configuration of Power Supply Circuit 10 FIG. 1 is a diagramillustrating a circuit configuration of a power supply circuit 10according to the first embodiment. The power supply circuit 10 is abidirectional DC/DC converter. That is, the power supply circuit 10 cantransmit power bi-directionally (i) from a low-voltage power supply to ahigh-voltage power supply, or (ii) from a high-voltage power supply to alow-voltage power supply. In the power supply circuit 10, a knownhalf-bridge circuit of the bidirectional DC/DC converter is replacedwith the half-bridge circuit 1 of the first embodiment.

Note that each of the numerical values described below is merely anexample.

Configuration of Low-Voltage Portion of Power Supply Circuit 10

A low-voltage portion is provided with a power supply LV1 (low-voltagepower supply) and a coil CO1. The voltage of the power supply LV1 is 200V. The (+) side of the DC power supply symbols in FIG. 1 indicates apositive pole side. The voltage of the (−) side is 0 V. An inductance ofthe coil CO1 is 500 μH. An average current of the coil CO1 in a steadystate is 14 A.

Configuration of High-Voltage Portion of Power Supply Circuit 10

A high-voltage portion is provided with a power supply HV1 (high-voltagepower supply) and a capacitor SC1. The voltage of the power supply HV1is 400 V. The capacitor SC1 has a capacitance of 3.3 mF and a voltage of400 V. In the power supply circuit 10, the voltage of the high-voltageportion is designed to be twice the voltage of the low-voltage portion.

Configuration of Half-Bridge Circuit 1 of Power Supply Circuit 10

A typical half-bridge circuit includes a first switching element FSW1and a second switching element SSW1. In the half-bridge circuit 1, atransformer TR1, a first rectifying element FR1, a second rectifyingelement SR1, a transistor element TTR1, and a power supply TV1 arefurther provided to the typical half-bridge circuit. As described later,the transformer TR1 includes three types of windings (primary totertiary windings).

Note that, in the following description, for purposes of simplicity ofdescription, “the first switching element FSW1” is simply referred to as“FSW1,” for example. The same applies to other such members as well.

The “first switching element FSW1” is a cascode GaN-HEMT connected tothe upper arm side. In particular, FSW1 is a switching element in whicha normally-ON GaN-HEMT is cascode-connected to a low-voltage Si-MOSFET.FSW1 has a drain withstand voltage of 650 V and an ON resistance of 50mΩ. In the example of FIG. 1, the same circuit symbol as that for theMOSFET is used to represent the cascode GaN-HEMT.

The “second switching element SSW1” is a GaN-HEMT connected to the lowerarm side. The type of GaN-HEMT used is the same as that of the GaN-HEMTconnected to the upper arm side. SSW1 is the same as FSW1 except for theconnection location.

The “first rectifying element FR1” is a SiC-SBD connected to the upperarm side. The reverse breakdown voltage of FR1 is 650 V. Further, theforward voltage of FR1 at a point in time when conduction starts is 0.9V. The resistance of FR1 when the forward current is flowing is 50 mΩ.FR1 is connected in parallel with FSW1 with a secondary winding SW1therebetween as described below. Herein, the connection relationshipbetween FSW1 and FR1, denoted by the symbol in FIG. 1, is defined as aparallel connection. Further, FR1 may also be connected to the sourceside of FSW1 by switching the order of FR1 with SW1.

The “second rectifying element SR1” is a SiC-SBD connected to the lowerarm side. SR1 is connected in parallel with SSW1 with a tertiary windingTW1 therebetween as described below. The element of SR1 used is the sametype as that of FR1. Further, SR1 may also be connected to the sourceside of SSW1 by switching the order of SR1 with TW1.

The “transformer TR1” includes the primary winding PW1, the secondarywinding SW1, and the tertiary winding TW1. The number of turns of PW1 isnine. The inductance of PW1 is 1.6 pH.

The resistance of PW1 is 10 mΩ. The inductance of PW1 is also referredto as excitation inductance. The number of turns of SW1 and TW1 is sixeach. The resistance of SW1 and TW1 is 7 mΩ each.

Further, in the half-bridge circuit 1, each component is connected sothat the polarity of SW1 from the second terminal toward the firstterminal and the polarity of TW1 from the fourth terminal toward thethird terminal are the same.

The “transistor element TTR1” is connected to PW1. The element of TTR1is the same as that of FSW1. However, TTR1 is used not as a switchingelement, but as a transistor element. The transistor element connectedto PW1 is also referred to herein as a “first transistor element.” TTR1is an example of a first transistor element.

The “gate terminal of each element” is connected to a control circuit 9described later (not illustrated in FIG. 1 or the like; refer to FIG.9). That is, the ON/OFF switching of the gate of each element isperformed by the control circuit 9. In this regard, the same applies tothe second and subsequent embodiments.

The “power supply TV1” is connected to PW1. The voltage of TV1 is 15 V.

The “first terminal FT1” refers to an electrical connection pointbetween a current path (hereinafter simply “path”) of FSW1 and the pathof FR1. Further, FT1 is set to a high-voltage node by the positiveelectrode of HV1. A high-voltage node is, for example, a node with avoltage from 100 V to 1000 V.

The “second terminal ST1” refers to an electrical connection pointbetween the path of FSW1 and the path of FR1. The ST1 is a switch nodein which the voltage is switched. A switch node is a node in which thevoltage varies due to circuit operation.

The “third terminal TT1” refers to an electrical connection pointbetween the path of SSW1 and the path of SR1. Note that TT1 is a switchnode similar to ST1. Accordingly, TT1 and ST1 may be connected togetherat the same connection point.

The “fourth terminal PT1” refers to an electrical connection pointbetween the path of SSW1 and the path of SR1. Further, PT1 is set to alow-voltage node by the negative electrode of HV1. The low-voltage nodeis a node having a lower voltage than the high-voltage node. In thefirst embodiment, the voltage of the low-voltage node is 0 V.

“FS1 and SS1” indicate a section where the rectified current and thetransient current on the upper arm side can be measured. That is,neither FS1 nor SS1 are current sensors. In both FS1 and SS1, the samecurrent values can be observed. Further, any current sensor can be usedas the current sensor. For example, a Hall element type current sensor,a Current Transformer (CT) sensor, a Rogowski coil and shunt resistancemethod, or the like can be used.

“TS1 and PS1” indicate a section where the rectified current and thetransient current on the lower arm side can be measured. Other aspectsare the same as those of FS1 and SS1. Configuration of Power SupplyCircuit 10 r of Comparative Example First, consider the operation of abidirectional DC/DC converter (hereinafter, a power supply circuit 10 r)of a comparative example. The power supply circuit 10 r is constitutedby the typical half-bridge circuit described above. In other words, inthe power supply circuit 10 r, TR1, FR1, SR1, TTR1, and TV1 have beenremoved from the power supply circuit 10 illustrated in FIG. 1. Therelationship between the operation of the power supply circuit 10 r andtransient current will be described in detail below.

First, the step-up operation will be described. The step-up operation isan operation in which power is transmitted from a low-voltage powersupply to a high-voltage power supply. In the step-up operation, FSW1 isused as a rectifying element and SSW1 is used as a transistor element.

Step-Up Operation 1 of Comparative Example

First, in the ON time period of SSW1, the voltage of the switch nodes(ST1, TT1) is approximately 0 V. Thus, a voltage of 200 V is applied toCO1, increasing the coil current. The coil current follows the path “LV1positive electrode→CO1→SSW1→LV1 negative electrode.”

Step-Up Operation 2 of Comparative Example

Then, SSW1 is switched from ON to OFF. As a result, due to theelectromotive force of CO1, the voltage of the switch node isapproximately 1 V greater than the voltage of FT1. This voltage ofapproximately 1 V is applied as a forward voltage to FSW1, and arectified current flows from CO1 to FSW1. The current follows the path“LV1 positive electrode→CO1→FSW1→HV1→LV1 negative electrode.” Power istransmitted to HV1 by this current.

Step-Up Operation 3 of Comparative Example

Then, SSW1 is switched from OFF to ON. As a result, the voltage of theswitch node is approximately 0 V. Thus, a reverse voltage ofapproximately 400 V is applied to FSW1. This reverse voltage of 400 Vcauses the parasitic capacitance of FSW1 to be charged and a transientcurrent to occur.

In the step-up operation time period, the step-up operations 1 to 3described above are repeatedly performed. The drive frequency of SSW1 is100 kHz, and SSW1 repeatedly turns ON and OFF with a duty ratio of 50%.Thus, every 5 μsec, the forward voltage and the reverse voltage arealternately applied to FSW1.

Step-Down Operation of Comparative Example

The step-down operation will now be described. The step-down operationis an operation in which power is transmitted from a high-voltage powersupply to a low-voltage power supply. In the step-down operation, FSW1is used as a transistor element and SSW1 is used as a rectifyingelement. First, FSW1 is turned ON, causing the current to flow along thepath “LV1 negative electrode→HV1→FSW1→CO1→LV1 positive electrode.” Then,FSW1 is turned OFF, causing the current to flow along the path “LV1negative electrode→SSW1→CO1→LV1 positive electrode.” Then, FSW1 isswitched from OFF to ON. As a result, a reverse voltage of approximately400 V is applied to SSW1, and a transient current occurs.

In this way, by switching the role (function) of the switching elementbetween the upper and lower arms, it is possible to make the half-bridgecircuit selectively execute a step-up operation or a step-downoperation.

Description of Drawing Used in Operation Explanation of First TimePeriod of Half-Bridge Circuit 1

The step-up operation of the half-bridge circuit 1 included in the powersupply circuit 10 will now be described. In the step-up operation, FSW1is used as a rectifying element (rectifier function unit) and SSW1 isused as a transistor element (transistor function unit). As such, thestep-up operation time period is an example of the first time period.

FIG. 2 is a graph showing waveforms of each voltage and current of thehalf-bridge circuit 1. FIG. 2 shows four waveforms on a common time axis(horizontal axis). Further, the timing of first to fourth stepsdescribed below is indicated on the horizontal axis in FIG. 2.

The voltages and currents shown in FIG. 2 are:

-   -   RFV (rectifier function unit voltage): Voltage applied to FT1        based on ST1,    -   RFI (rectifier function unit current): Current flowing from ST1        to FT1,    -   PW1I (primary winding current): Current flowing from TV1 to PW1,        and    -   FR1I (first rectifying element current): Forward current of FR1.

FIG. 3 is a graph showing each graph of FIG. 2 enlarged. In FIG. 3,unlike FIG. 2, the four waveforms are shown in one graph. Note that, inFIG. 3, for convenience of the enlarged view, RFV extends beyond theupper end of the graph.

FIG. 4 is a diagram for explaining the path of each current in the firstto fourth steps in the first time period. Specifically, (a) to (d) ofFIG. 4 are diagrams for explaining the path of each current in the firstto fourth steps in the first time period, respectively. For convenienceof illustration, in FIG. 4, the reference numerals of the elements inFIG. 1 are omitted. Also, in FIG. 4, the illustration of each element issimplified in comparison to FIG. 1.

Drive Method in First Time Period of Half-Bridge Circuit 1: First Stepto Fourth Step

According to the drive method of the first time period of thehalf-bridge circuit 1, the four steps below are executed in order. Inthe following, each step is described in detail.

-   -   First step: Applying a forward voltage to FSW1 and thus causing        a rectified current to flow    -   Second step: Turning TTR1 ON and thus causing a current to flow        to PW1    -   Third step: Turning TTR1 OFF and thus causing a current to flow        to FR1    -   Fourth step: Applying a reverse voltage to FSW1 and thus        stopping the rectified current        First Step: Causing Rectified Current to Flow to Rectifier        Function Unit

Prior to the first step, current is flowing from CO1 to SSW1. Thus, inthe first step, SSW1 is turned OFF, generating an electromotive force inCO1. This electromotive force allows a forward voltage of approximately1 V to be applied to FSW1. As a result, a rectified current (RFI) can becaused to flow to FSW1. In other words, in the first step, FSW1 is usedas a rectifier function unit. RFI flows through the path illustrated in(a) of FIG. 4.

Note that, in the first step, the size of the current flowing to FR1 isless than the size of the current flowing to FSW1. Thus, in (a) of FIG.4, unlike (c) to (d) of FIG. 4, FR1I is not illustrated.

Second Step: Causing Current to Flow to Primary Winding

Following the first step, TTR1 is turned ON. This allows PW1I to flow.PW1I flows through the path illustrated in (b) of FIG. 4. In the secondstep, PW1I increases substantially linearly over time.

Third Step: Causing Current to Flow to Rectifying Element Following thesecond step, TTR1 is turned OFF, causing PW1I to be approximately 0 A.This allows FR1I to flow. FR1I flows through the path illustrated in (c)of FIG. 4. As illustrated in (c) of FIG. 4, FR1I flows from SW1 to FR1.

This current path of FR1I may also be described from another standpoint.In particular, the current flowing to FSW1 in (c) of FIG. 4 will bedescribed. (c) of FIG. 4 illustrates both RFI (facing upward at theposition of FSW1 in the drawing) and FR1I (facing downward at theposition of FSW1 in the drawing) in FSW1.

The two currents in directions opposite to each other flowing to FSW 1at the same timing means that subtraction of the two current valuesoccurs in FSW1.

Fourth Step: Applying Reverse Voltage to Rectifier Function UnitFollowing the third step, a reverse voltage is applied to FSW1. In thefourth step, SSW1 is turned ON, and thus used as a transistor functionunit. Turning SSW1 ON allows a reverse voltage to be applied to FSW1.The method of applying the reverse voltage may be selected from avariety of methods according to the type of power supply circuit. Thereverse voltage should thus be applied using a method corresponding tothe various power supply circuits.

Concurrent with the application of the reverse voltage, a transientcurrent (RFI in the reverse direction) that charges the parasiticcapacitance of the FSW1 occurs. The transient current flows along thepath indicated by RFI in (d) of FIG. 4.

In addition, although not illustrated in (d) of FIG. 4, current flowsalong the path “LV1 positive electrode→CO1→SSW1→LV1 negative electrode”from the start point of the fourth step.

Principle of Transient Current Reduction in Fourth Step

The current that charges the parasitic capacitance of FSW1 is not onlyRFI in the reverse direction. The FR1I flowing in the third step flowsalong a path that charges the parasitic capacitance of FSW1 (refer to(d) of FIG. 4). That is, the parasitic capacitance can be charged byFR1I and RFI. Thus, the transient current is a value obtained bysubtracting an amount equivalent to FR1I. That is, the transient currentcan be effectively reduced compared to the related art.

Connection Format of Transformer Capable of Storing and ReleasingMagnetic Energy

TR1 is a member for storing and releasing magnetic energy. Thus, TR1 isconfigured in a connection format that allows both the storage andrelease of the magnetic energy described below.

Magnetic Energy Storage

In a case that a positive voltage is applied to a black dot side basedon a non-black dot side of PW1, then a positive voltage occurs in SW1 onthe black dot side based on the non-black dot side. However, in thehalf-bridge circuit 1, FR1 is interposed in the path from the black dotside to the non-black dot side of SW1. Thus, a reverse voltage isapplied to FR1, and FR1I does not flow.

Further, in the half-bridge circuit 1, SR1 is interposed in the pathfrom the black dot side to the non-black dot side of TW1. Even in SR1, arectifying element current does not flow. Accordingly, in the secondstep, the magnetic energy originating from PW1I can be stored in TR1.

Magnetic Energy Release

By the blocking of PW1I in the third step, the polarity of the voltageapplied to SW1 is reversed. Thus, a forward voltage is applied to FR1,and FR1I flows.

Furthermore, in the half-bridge circuit 1, the polarity of the voltageapplied to TW1 is also reversed. However, in the third step, since SSW1is OFF, a high voltage of approximately 400 V is applied to the thirdterminal. Thus, SR1 does not cause current to flow. Thus, by theblocking of PW1I in the third step, FR1I is caused to flow.

As described above, in the half-bridge circuit according to an aspect ofthe present disclosure, a connection relationship of the transformer isset so that PW1I and FR1I are not caused to flow at the same time. Thus,the presence or absence of a black dot (polarity) of each of the primarywinding to the tertiary winding may be designed in reverse.

Further, the blocking of PW1I causes one of the following results: (i)“the current flows to SW1 so as to pass through FSW1,” or (ii) “thecurrent flows to TW1 so as to pass through SSW1.” In the step-upoperation in the first embodiment, because the voltage of FSW1 is low,“the current flows to SW1 so as to pass through FSW1.” In contrast, inthe case of the step-down operation, “the current flows to TW1 so as topass through SSW1.”

Description of Effect of Reducing Transient Current

With reference to FIG. 3 and FIG. 5, the effect of reducing thetransient current in the half-bridge circuit 1 will now be described.FIG. 5 is a graph showing waveforms of a rectifier function unit voltage(RFVc) and a rectifier function unit current (RFIc) in the power supplycircuit 10 r. The scales of the horizontal axis and vertical axis in thegraph of FIG. 5 are set to the same scales as in the graph in FIG. 3.

1. Effect of Reducing Transient Current

Comparative Example

The transient current of FSW1, which is the rectifier function unit ofthe power supply circuit 10 r, will now be described with reference toFIG. 5. When RFVc, as the reverse voltage, is applied at 400 V, atransient current (negative RFIc) flows (refer to at or near time“1.053E-5 sec”). In FIG. 5, voltage over 30 V is not shown forconvenience of the scale of the vertical axis. However, RFVc is at orgreater than 400 V. Thus, in the power supply circuit 10 r, a transientcurrent having a size of approximately 27 A flows.

Half-Bridge Circuit 1

The transient current in the half-bridge circuit 1 will now be describedwith reference to FIG. 3. Similar to the comparative example, a reversevoltage (RFV) of 400 V is applied (refer to at or near the same time asin the comparative example in FIG. 5). Nevertheless, the size of thetransient current (negative RFI) is approximately 19 A. As describedabove, according to the half-bridge circuit 1, it is confirmed that thetransient current can be reduced compared to that in the comparativeexample.

Improvements 1 to 4 for Efficiently Operating Half-Bridge Circuit 1

In the first embodiment, a plurality of preferred improvements areapplied. These preferred improvements will be described below.

Improvement 1: Rectifying Element Current Caused to Flow When RectifiedCurrent is Flowing to Rectifier Function Unit

As described above, FR1I is used for transient current reduction. Thus,it is also important to suppress attenuation of FR1I during the timeperiod until the transient current flows. In the first embodiment, FR1Iis flowing when a rectified current flows to FSW1. When current isflowing to FSW1, the voltage of the first terminal relative to thesecond terminal decreases by an amount equivalent to the voltage drop ofFSW1. On the other hand, FR1I flows from the second terminal towards thefirst terminal via SW1 and FR1. In other words, the current is caused toflow from the second terminal having a higher voltage to the firstterminal having a lower voltage. Therefore, attenuation of FR1I can besuppressed.

Improvement 2: Increase in Inductance of Flow Path of Rectifying ElementCurrent

As described above, FR1I is used for transient current reduction. Thus,it is also important that FR1I continually flows when the transientcurrent is flowing. In the first embodiment, the inductance (hereinafterL12 a) of the path from the first terminal to the second terminal viaFSW1 is set greater than the inductance (hereinafter L12 b) of the pathfrom the first terminal to the second terminal via FR1. Thus, FR1I canbe caused to continuously flow by L12 b during a time period when atransient current is flowing to FSW1. In the first embodiment, L12 a is10 nH, and L12 b is 100 nH. For example, L12 b is preferably at leasttwo times L12 a. Further, L12 b is more preferably at least ten timesL12 a.

Improvement 3: Increase in Turns of Primary Winding

A conduction loss occurs in each winding of TR1. In the firstembodiment, conduction loss is reduced by reducing the current of PW1.

The winding current of a transformer is, in principle, inverselyproportional to the turn ratio. In the first embodiment, the number ofturns of SW1 and TW1 are each set to six. On the other hand, the numberof turns of PW1 is set to nine. In other words, the number of turns ofPW1 is set greater than each of the number of turns of SW1 and TW1. As aresult, the amount of current of PW1 can be reduced while maintainingthe current of SW1 or TW1. The reduction in the amount of current makesit possible to reduce conduction loss.

Improvement 4: Magnetic Energy Ensured on Secondary Winding Side

As described above, FR1I is used for transient current reduction. TheFR1I is generated by the magnetic energy of TR1. Thus, storage of agreater amount of magnetic energy in TR1 leads to further reduction intransient current. In the first embodiment, improvements are made tostore a greater amount of magnetic energy in TR1.

Specifically, in the first embodiment, the voltage drop amount of FSW1generated by current flowing from the second terminal to the firstterminal is set greater than the voltage drop amount at a point in timewhen FR1 conduction starts.

Such a configuration allows current to flow to SW1 via FR1 when currentis flowing to FSW1. This current can be verified in FR1I in FIG. 2. Asshown in FIG. 2, at time “1.00E-5 sec,” approximately 2 A of FR1I isflowing. Thus, it is understood that PW1I at that same time increasesnot from 0 A, but from a start point (initial value) of approximately 2A. In the first embodiment, such storage of magnetic energy is alsoperformed.

Description of Drawing Used in Operation Explanation of Second TimePeriod of Half-Bridge Circuit 1

The step-down operation of the half-bridge circuit 1 is described below.In the step-down operation, SSW1 is used as a rectifying element(rectifier function unit) and FSW1 is used as a transistor element(transistor function unit). As such, the step-down operation time periodis an example of the second time period.

FIG. 6 is a diagram for explaining the path of each current in the firstto fourth steps in the second time period. FIG. 6 is a diagram pairedwith FIG. 4 (description of the step-up operation). FIG. 6 illustratesthe path of each current in the first to fourth steps. Specifically, (a)to (d) of FIG. 6 illustrate the path of each current in the first tofourth steps, respectively. RFI is the current of the rectifier functionunit (SSW1 in the case of FIG. 6). SR1I is the current of the secondrectifying element (second rectifying element current). As illustratedin (c) of FIG. 6, SR1I flows from TW1 to SR1.

Drive Method in Second Time Period of Half-Bridge Circuit 1: First Stepto Fourth Step

According to the drive method of the second time period of thehalf-bridge circuit 1, the four steps below are executed in order.

-   -   First step: Applying a forward voltage to SSW1 and thus causing        a rectified current to flow    -   Second step: Turning TTR1 ON and thus causing a current to flow        to PW1    -   Third step: Turning TTR1 OFF and thus causing a current to flow        to SR1    -   Fourth step: Applying a reverse voltage to SSW1 and thus        stopping the rectified current

In the half-bridge circuit 1, in the step-down operation, the rectifierfunction unit and the switch function unit are switched between theupper and lower arms relative to the step-up operation. The orientationof the CO1 current is also reversed. These matters are the same as inthe half-bridge circuit in the related art. Therefore, although the armin which the transient current is generated differs, the effect ofreducing the transient current in the step-down operation is the same asthe effect of reducing the transient current in the step-up operation.This is because the upper and lower arms have the same configurationbased on the same elements.

For example, on the basis of improvement 2 described above, theinductance of the path from the fourth terminal to the third terminalvia the second rectifying element is set to be at least two timesgreater than the inductance of the path from the fourth terminal to thethird terminal via the second switching element.

Further, on the basis of improvement 4 described above, the voltage dropamount of the second switching element generated by the current flowingfrom the fourth terminal to the third terminal is set greater than thevoltage drop amount at a point in time when the second rectifyingelement starts conduction.

Thus, although not illustrated, the amount of transient currentreduction in the step-down operation is also the same as the amount oftransient current reduction in the step-up operation.

Consider a half-bridge circuit including a first time period and asecond time period as operation time periods. In the half-bridgecircuit, (i) FSW1 and SSW1 are used as a rectifier function unit and atransistor function unit, respectively, in the first time period, and(ii) FSW1 and SSW1 are used as a transistor function unit and arectifier function unit, respectively, in the second time period. Inthis case, by applying the half-bridge circuit 1 of the first embodimentas the half-bridge circuit, it is possible to reduce the transientcurrent in both the first time period and the second time period by onetransformer (TR1).

Modified Example: Scope of Application of Element

In the first embodiment, an example is given in which FSW1 and SSW1 arecascode GaN-HEMTs and FR1 and SR1 are SiC-SBDs. The types of theseelements are not particularly limited to a specific type as long as theyfall within the scope of the elements described above. Similarly, thetype of TTR1 is not particularly limited to a specific type as long asthe type has a transistor function.

Second Embodiment

FIG. 7 is a diagram illustrating a circuit configuration of a powersupply circuit 20 according to the second embodiment. The power supplycircuit 20 includes a half-bridge circuit 2. In the half-bridge circuit2, TV1 of the half-bridge circuit 1 is replaced with LV1. In otherwords, in the power supply circuit 20, LV1 also serves as a power supplyof the half-bridge circuit 2. According to this configuration, the totalnumber of power supplies in the power supply circuit 20 can be reduced,which is advantageous in terms of cost.

In the half-bridge circuit 2, a transformer TR2 is provided in place ofTR1 of the half-bridge circuit 1. TR2 includes a primary winding PW2, asecondary winding SW2, and a tertiary winding TW2.

Further, in the half-bridge circuit 2, transistor elements TTR2, TTR3,TTR4 are provided in place of TTR1 of the half-bridge circuit 1. Thehalf-bridge circuit 2 is a modified example of the circuit configurationon the primary winding side with respect to the half-bridge circuit 1.TTR2, TTR3, and TTR4 all function as transistor elements.

With such a circuit configuration as well, turning TTR2 and TTR3 ONallows storage of magnetic energy in TR2, similar to the firstembodiment. Thus, the transient current can be reduced.

Third Embodiment

FIG. 8 is a diagram illustrating a circuit configuration of a powersupply circuit 30 according to the third embodiment. The power supplycircuit 30 includes a half-bridge circuit 3. In the half-bridge circuit3, TV1 of the half-bridge circuit 1 is replaced with HV1. According tothis configuration as well, the total number of power supplies in thepower supply circuit 30 can be reduced, which is advantageous in termsof cost.

In the half-bridge circuit 3, a transformer TR3 is provided in place ofTR1 of the half-bridge circuit 1. TR3 includes a primary winding PW3, asecondary winding SW3, and a tertiary winding TW3.

Further, in the half-bridge circuit 3, transistor elements TTR5, TTR6,TTR7 are provided in place of TTR1 of the half-bridge circuit 1. Thehalf-bridge circuit 3 is also a modified example of the circuitconfiguration on the PW1 side with respect to the half-bridge circuit 1.TTR5, TTR6, and TTR7 also all function as transistor elements.

With such a circuit configuration as well, turning TTR5 and TTR6 ONallows storage of the magnetic energy in TR3, similar to the firstembodiment, and thus the transient current can be reduced.

Fourth Embodiment

The half-bridge circuit according to an aspect of the present disclosureexhibits a higher effect with respect to a power supply circuitincluding the first time period and the second time period as theoperation time periods. Examples of the power supply circuit include abidirectional chopper circuit, an inverter circuit, a totem pole PowerFactor Correction (PFC) circuit, and the like.

FIG. 9 is a diagram illustrating a power supply device 100 provided withthe power supply circuit 10 (power supply circuit including thehalf-bridge circuit 1). According to the half-bridge circuit 1, loss inthe power supply circuit 10 and the power supply device 100 can bereduced. Furthermore, the power supply device 100 includes the controlcircuit 9. The control circuit 9 controls each component of the powersupply circuit 10. More specifically, the control circuit 9 controls theON/OFF switching of the elements provided to the power supply circuit10. The first to fourth steps may be performed by the control circuit 9controlling the ON/OFF of each switching element provided to the powersupply circuit 10.

Supplement

A half-bridge circuit according to a first aspect of the presentdisclosure includes a first switching element connected to a firstterminal serving as a high-voltage node and a second terminal serving asa switch node and a second switching element connected to a thirdterminal serving as a switch node and a fourth terminal serving as alow-voltage node. The half-bridge circuit further includes a transformerincluding a primary winding, a secondary winding, and a tertiarywinding, a first rectifying element connected in parallel with the firstswitching element with the secondary winding interposed between thefirst rectifying element and the first switching element, a secondrectifying element connected in parallel with the second switchingelement with the tertiary winding between the second rectifying elementand the second switching element, a first transistor element connectedto the primary winding, and a power supply connected to the primarywinding. In a case that the first transistor element is turned ON, aprimary winding current, serving as a current flowing from the powersupply to the primary winding, flows, and in a case that the firsttransistor element is turned OFF, (i) a first rectifying elementcurrent, serving as a current flowing from the secondary winding to thefirst rectifying element flows, or (ii) a second rectifying elementcurrent, serving as a current flowing from the tertiary winding to thesecond rectifying element, flows.

As described above, a transient current generates a loss in a circuit.Thus, the inventors of the present application discovered theconfiguration described above on the basis of the idea that “causing acurrent generated by magnetic energy stored in a transformer to flow toeach switching element leads to suppression of transient current.”

According to the configuration described above, by turning the firsttransistor element ON, it is possible to cause a current to flow to theprimary winding and store magnetic energy in the transformer. Then, byturning the first transistor element OFF, it is possible to convert themagnetic energy to the first rectifying element current, and cause thefirst rectifying element current to flow to the first switching element.In this first rectifying element current, the current component thatbecomes the transient current of the first switching element is causedto flow through an internal path formed from the secondary winding, thefirst rectifying element, and the first switching element. Thus, thetransient current flowing to the first switching element can be reduced.

The above description of the secondary winding, the first rectifyingelement, and the first switching element also applies to the tertiarywinding, the second rectifying element, and the second switchingelement. Accordingly, in the same way as described above, the transientcurrent flowing to the second switching element can also be reduced.

As described above, according to the half-bridge circuit according to anaspect of the present disclosure, it is possible to effectively reducetransient current in both the first switching element and the secondswitching element. That is, the transient current can be moreeffectively reduced than in the related art.

In the half-bridge circuit according to a second aspect of the presentdisclosure, in the first aspect, preferably a polarity of the secondarywinding from the second terminal toward the first terminal is the sameas a polarity of the tertiary winding from the fourth terminal towardthe third terminal.

According to the configuration (the winding configuration of thetransformer) described above, of the first switching element and thesecond switching element, the rectifying element current (the firstrectifying element current or the second rectifying element current) canbe selectively caused to flow to the switching element having a lowerapplied voltage. That is, the rectifying element current can beselectively caused to flow to a predetermined switching element to whicha reverse voltage will be applied (a switching element in which atransient current is expected to occur).

In the half-bridge circuit according to a third aspect of the presentdisclosure, in the first or the second aspect, preferably the firstswitching element is used as a rectifier function unit and the secondswitching element is used as a transistor function unit in a first timeperiod, and the first switching element is used as a transistor functionunit and the second switching element is used as a rectifier functionunit in a second time period.

According to the configuration described above, the switching elementused as the rectifier function unit in the first time period and thesecond time period can be switched. In other words, the transientcurrent can be selectively generated in the first switching element orthe second switching element, in accordance with the time period. Inthis case, with use of the transformer described above, the rectifyingelement current can be selectively caused to flow to the switchingelement in which the transient current flows.

In the half-bridge circuit according to a fourth aspect of the presentdisclosure, in the third aspect, preferably conduction of the firstrectifying element current is started within a time period when currentis flowing from the second terminal to the first terminal via the firstswitching element in the first time period, and conduction of the secondrectifying element current is started within a time period when currentis flowing from the fourth terminal to the third terminal via the secondswitching element in the second time period.

According to the configuration described above, in the first timeperiod, the voltage of the second terminal is greater than the voltageof the first terminal by an amount equivalent to the voltage drop amountof the first switching element. Thus, the first rectifying elementcurrent can be caused to flow from the second terminal (the terminal onthe high-voltage side) toward the first terminal (the terminal on thelow-voltage side), allowing the first rectifying element current tocontinuously flow.

Further, in the same way as described above for the first time period,the second rectifying element current can be caused to continuously flowin the second time period. As a result, the effect of reducing thetransient current can be further enhanced for both the switchingelements in the half-bridge circuit.

In the half-bridge circuit according to a fifth aspect of the presentdisclosure, in any one of the first to fourth aspects, preferably aninductance of a path from the second terminal to the first terminal viathe first rectifying element is at least two times greater than aninductance of a path from the second terminal to the first terminal viathe first switching element, and an inductance of a path from the fourthterminal to the third terminal via the second rectifying element is atleast two times greater than an inductance of a path from the fourthterminal to the third terminal via the second switching element.

According to the configuration described above, the inductance of thepath via the first rectifying element is sufficiently greater than theinductance of the path via the first switching element, thereby causingthe first rectifying element current that was once conducted tocontinuously flow.

Further, in the same way as described above for the first rectifyingelement current, the second rectifying element current can also becaused to continuously flow. As a result, the effect of reducing thetransient current can be further enhanced for both the switchingelements in the half-bridge circuit.

In the half-bridge circuit according to a sixth aspect of the presentdisclosure, in any one of the first to fifth aspects, preferably anumber of turns of the primary winding is greater than a number of turnsof each of the secondary winding and the tertiary winding.

As described above, the winding current of a transformer is inverselyproportional to the turn ratio. Thus, according to the configurationdescribed above, the amount of current in the primary winding can bereduced while maintaining the amounts of current of each of thesecondary winding and the tertiary winding. Thus, a reduction inconduction loss is possible.

In the half-bridge circuit according to a seventh aspect of the presentdisclosure, in any one of the first to sixth aspects, preferably avoltage drop amount of the first switching element generated by acurrent flowing from the second terminal to the first terminal isgreater than a voltage drop amount of the first rectifying element at apoint in time when conduction starts, and a voltage drop amount of thesecond switching element generated by a current flowing from the fourthterminal to the third terminal is greater than a voltage drop amount ofthe second rectifying element at a point in time when conduction starts.

According to the configuration described above, when current is flowingto the first switching element, the current can be caused to flow to thesecondary winding via the first rectifying element. Thus, it is possibleto store magnetic energy from the secondary winding side to thetransformer.

Further, in the same way as described above for the first switchingelement, the first rectifying element, and the secondary winding, it isalso possible to store magnetic energy from the tertiary winding side tothe transformer. As a result, the effect of reducing the transientcurrent can be further enhanced.

A power supply circuit according to an eighth aspect of the presentdisclosure preferably includes the half-bridge circuit according to anyone of the first to seventh aspects.

According to the configuration described above, a power supply devicewith reduced loss can be realized by using the half-bridge circuit witha reduced transient current.

A method for driving a half-bridge circuit according to a ninth aspectof the present disclosure is a method for driving a half-bridge circuitincluding a first switching element connected to a first terminalserving as a high-voltage node and a second terminal serving as a switchnode and a second switching element connected to a third terminalserving as a switch node and a fourth terminal serving as a low-voltagenode. The half-bridge circuit further includes a transformer including aprimary winding, a secondary winding, and a tertiary winding, a firstrectifying element connected in parallel with the first switchingelement with the secondary winding interposed between the firstrectifying element and the first switching element, a second rectifyingelement connected in parallel with the second switching element with thetertiary winding interposed between the second rectifying element andthe second switching element, a first transistor element connected tothe primary winding, and a power supply connected to the primarywinding. The method includes a first step for applying a forward voltageto the first switching element and causing a rectified current to flowto the first switching element, a second step for turning the firsttransistor element ON after the first step and causing a primary windingcurrent, serving as a current flowing from the power supply to theprimary winding, to flow, a third step for turning the first transistorelement OFF after the second step and causing a first rectifying elementcurrent, serving as a current flowing from the secondary winding to thefirst rectifying element, to flow, and a fourth step for applying areverse voltage to the first switching element after the third stepwithin a time period when the first rectifying element current isflowing.

According to the configuration described above, the same effects asthose of the half-bridge circuit according to an aspect of the presentdisclosure are achieved.

APPENDIX

The method for driving a half-bridge circuit according to an aspect ofthe present disclosure can also be expressed as follows.

That is, a method for driving a half-bridge circuit according to anaspect of the present disclosure is a method for driving a half-bridgecircuit including a first switching element connected to a firstterminal serving as a high-voltage node and a second terminal serving asa switch node and a second switching element connected to a thirdterminal serving as a switch node and a fourth terminal serving as alow-voltage node. The half-bridge circuit further includes a transformerincluding a primary winding, a secondary winding, and a tertiarywinding, a first rectifying element connected in parallel with the firstswitching element with the secondary winding interposed between thefirst rectifying element and the first switching element, a secondrectifying element connected in parallel with the second switchingelement with the tertiary winding interposed between the secondrectifying element and the second switching element, a first transistorelement connected to the primary winding, and a power supply connectedto the primary winding. The method includes a first step (A) forapplying a forward voltage to the first switching element and causing arectified current to flow to the first switching element, a second step(A) for turning the first transistor element ON after the first step (A)and causing a primary winding current, serving as a current flowing fromthe power supply to the primary winding, to flow, a third step (A) forturning the first transistor element OFF after the second step (A) andcausing a first rectifying element current, serving as a current flowingfrom the secondary winding to the first rectifying element, to flow, anda fourth step (A) for applying a reverse voltage to the first switchingelement after the third step (A) within a time period when the firstrectifying element current is flowing, or a first step (B) for applyinga forward voltage to the second switching element and causing arectified current to flow to the second switching element, a second step(B) for turning the first transistor element ON after the first step (B)and causing the primary winding current to flow from the power supply tothe primary winding, a third step (B) for turning the first transistorelement OFF after the second step (B) and causing a second rectifyingelement current, serving as a current flowing from the tertiary windingto the second rectifying element, to flow, and a fourth step (B) forapplying a reverse voltage to the second switching element after thethird step (B) within a time period when the second rectifying elementcurrent is flowing.

Supplementary Information

An aspect of the present disclosure is not limited to each of theabove-described embodiments. It is possible to make variousmodifications within the scope of the claims. An embodiment obtained byappropriately combining technical elements each disclosed in differentembodiments falls also within the technical scope of the presentdisclosure. Furthermore, technical elements disclosed in the respectiveembodiments may be combined to provide a new technical feature.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

The invention claimed is:
 1. A half-bridge circuit comprising: a firstswitching element connected to a first terminal serving as ahigh-voltage node and a second terminal serving as a switch node; and asecond switching element connected to a third terminal serving as aswitch node and a fourth terminal serving as a low-voltage node, whereinthe first switching element and the second switching element areconnected in series, wherein the half-bridge circuit further includes atransformer including a primary winding, a secondary winding, and atertiary winding, a capacitor connected to the first terminal and thefourth terminal, a first rectifying element connected in parallel withthe first switching element with the secondary winding interposedbetween the first rectifying element and the first switching element, asecond rectifying element connected in parallel with the secondswitching element with the tertiary winding interposed between thesecond rectifying element and the second switching element, a firsttransistor element connected to the primary winding, and a power supplyconnected to the primary winding, in a case that the first transistorelement is turned ON, a primary winding current, serving as a currentflowing from the power supply to the primary winding, flows, and acurrent flowing through each of the secondary winding and the tertiarywinding is suppressed by the first rectifying element or the secondarywinding, in a case that the first transistor element is turned OFF, acurrent flowing from the power supply to the primary winding issuppressed by the first transistor element, and (i) a first rectifyingelement current, serving as a current flowing from the secondary windingto the first rectifying element, flows or (ii) a second rectifyingelement current, serving as a current flowing from the tertiary windingto the second rectifying element, flows.
 2. The half-bridge circuitaccording to claim 1, wherein a polarity of the secondary winding fromthe second terminal toward the first terminal is the same as a polarityof the tertiary winding from the fourth terminal toward the thirdterminal.
 3. The half-bridge circuit according to claim 2, wherein thefirst switching element is used as a rectifier function unit and thesecond switching element is used as a transistor function unit in afirst time period, and the first switching element is used as atransistor function unit and the second switching element is used as arectifier function unit in a second time period.
 4. The half-bridgecircuit according to claim 3, wherein conduction of the first rectifyingelement current is started within a time period when current is flowingfrom the second terminal to the first terminal via the first switchingelement in the first time period, and conduction of the secondrectifying element current is started within a time period when currentis flowing from the fourth terminal to the third terminal via the secondswitching element in the second time period.
 5. The half-bridge circuitaccording to claim 4, wherein an inductance of a path from the secondterminal to the first terminal via the first rectifying element is atleast two times greater than an inductance of a path from the secondterminal to the first terminal via the first switching element, and aninductance of a path from the fourth terminal to the third terminal viathe second rectifying element is at least two times greater than aninductance of a path from the fourth terminal to the third terminal viathe second switching element.
 6. The half-bridge circuit according toclaim 3, wherein an inductance of a path from the second terminal to thefirst terminal via the first rectifying element is at least two timesgreater than an inductance of a path from the second terminal to thefirst terminal via the first switching element, and an inductance of apath from the fourth terminal to the third terminal via the secondrectifying element is at least two times greater than an inductance of apath from the fourth terminal to the third terminal via the secondswitching element.
 7. The half-bridge circuit according to claim 2,wherein an inductance of a path from the second terminal to the firstterminal via the first rectifying element is at least two times greaterthan an inductance of a path from the second terminal to the firstterminal via the first switching element, and an inductance of a pathfrom the fourth terminal to the third terminal via the second rectifyingelement is at least two times greater than an inductance of a path fromthe fourth terminal to the third terminal via the second switchingelement.
 8. The half-bridge circuit according to claim 1, wherein thefirst switching element is used as a rectifier function unit and thesecond switching element is used as a transistor function unit in afirst time period, and the first switching element is used as atransistor function unit and the second switching element is used as arectifier function unit in a second time period.
 9. The half-bridgecircuit according to claim 8, wherein conduction of the first rectifyingelement current is started within a time period when current is flowingfrom the second terminal to the first terminal via the first switchingelement in the first time period, and conduction of the secondrectifying element current is started within a time period when currentis flowing from the fourth terminal to the third terminal via the secondswitching element in the second time period.
 10. The half-bridge circuitaccording to claim 9, wherein an inductance of a path from the secondterminal to the first terminal via the first rectifying element is atleast two times greater than an inductance of a path from the secondterminal to the first terminal via the first switching element, and aninductance of a path from the fourth terminal to the third terminal viathe second rectifying element is at least two times greater than aninductance of a path from the fourth terminal to the third terminal viathe second switching element.
 11. The half-bridge circuit according toclaim 8, wherein an inductance of a path from the second terminal to thefirst terminal via the first rectifying element is at least two timesgreater than an inductance of a path from the second terminal to thefirst terminal via the first switching element, and an inductance of apath from the fourth terminal to the third terminal via the secondrectifying element is at least two times greater than an inductance of apath from the fourth terminal to the third terminal via the secondswitching element.
 12. The half-bridge circuit according to claim 1,wherein an inductance of a path from the second terminal to the firstterminal via the first rectifying element is at least two times greaterthan an inductance of a path from the second terminal to the firstterminal via the first switching element, and an inductance of a pathfrom the fourth terminal to the third terminal via the second rectifyingelement is at least two times greater than an inductance of a path fromthe fourth terminal to the third terminal via the second switchingelement.
 13. The half-bridge circuit according to claim 1, wherein anumber of turns of the primary winding is greater than a number of turnsof each of the secondary winding and the tertiary winding.
 14. Thehalf-bridge circuit according to claim 1, wherein a voltage drop amountof the first switching element generated by a current flowing from thesecond terminal to the first terminal is greater than a voltage dropamount of the first rectifying element at a point in time whenconduction starts, and a voltage drop amount of the second switchingelement generated by a current flowing from the fourth terminal to thethird terminal is greater than a voltage drop amount of the secondrectifying element at a point in time when conduction starts.
 15. Apower supply device comprising: the half-bridge circuit described inclaim
 1. 16. A method for driving a half-bridge circuit including afirst switching element connected to a first terminal serving as ahigh-voltage node and a second terminal serving as a switch node and asecond switching element connected to a third terminal serving as aswitch node and a fourth terminal serving as a low-voltage node, whereinthe first switching element and the second switching element areconnected in series, wherein the half-bridge circuit further includes atransformer including a primary winding, a secondary winding, and atertiary winding, a capacitor connected to the first terminal and thefourth terminal, a first rectifying element connected in parallel withthe first switching element with the secondary winding interposedbetween the first rectifying element and the first switching element, asecond rectifying element connected in parallel with the secondswitching element with the tertiary winding interposed between thesecond rectifying element and the second switching element, a firsttransistor element connected to the primary winding, and a power supplyconnected to the primary winding, the method comprising: a first stepfor applying a forward voltage to the first switching element andcausing a rectified current to flow to the first switching element; asecond step for turning the first transistor element ON after the firststep and thereby causing a current flowing through each of the secondarywinding and the tertiary winding to be suppressed by the firstrectifying element or the secondary winding as well as causing a primarywinding current, serving as a current flowing from the power supply tothe primary winding, to flow; a third step for turning the firsttransistor element OFF after the second step and thereby causing acurrent flowing from the power supply to the primary winding to besuppressed by the first transistor element as well as causing a firstrectifying element current, serving as a current flowing from thesecondary winding to the first rectifying element, to flow; and a fourthstep for applying a reverse voltage to the first switching element afterthe third step within a time period when the first rectifying elementcurrent is flowing.